Known methods for measuring particle size include the use of electron microscopes, Coulter counters, centrifuges and dynamic light-scattering devices. However, all of these known methods require the removal, and subsequent dilution of the sample prior to testing, making such methods unsuited to on-line monitoring of particle size.
Known methods for measuring particle charge all involve the measurement of the particle velocity in an electric field. In some methods, the velocity is determined by measuring the time required for the particle to pass between two points on a microscopic grid when a steady electric field is applied. In other methods, particle velocity is measured by a light-scattering technique with an alternating electric field of 10 or 20 Hz.
There is apparatus suitable for determining both particle size and charge. The "Zeta Sizer" (Malvern Instruments) measures the light scattered from a laser beam as it passes through the suspension. In the absence of an applied field, the fluctuations in the scattered light provide information about the particle size, and if a slowly varying electric field is applied the particle charge can be obtained from the fluctuating light signal.
All of the above methods suffer the disadvantage that they require sample dilutions and most require use of light-scattering instruments. Accordingly, opaque samples cannot be measured.